Method for correcting seismic data to a datum

Abstract: Disclosed is a method for processing geophysical data that have been gathered over irregular topography. According to the method, a datum is established that lies above the highest topographic peak. The data are time-shifted from the surface to the datum using a replacement velocity that is an extrapolation of the underlying formation velocity. The wavefields beneath the surface are forwardly modeled using a modeling velocity substantially equal to zero. Between the surface and the datum, the wavefields are upwardly modeled using the replacement velocity.

Description: This invention relates to seismic data processing. Ordinarily, geophysical exploration of large areas is most readily conducted by emplacing the physical-measurement instrumentation on the surface of the earth. In some land areas, such as the Front Range of the Rocky Mountains, the topographic relief is a bit irregular. In other land areas, such as the Alberta syncline, the surface may be flat prairie but the underlying Paleozoic surface is severely eroded. Devonian reefs form a disconformity with respect to the underlying Silurian formation. At sea, the surface is usually flat but the sea floor may be topographically rugged with a substantial velocity contrast across the water/formation boundary. Unless properly corrected, the raw geophysical measurements are distorted by the irregular topography of the earth at the surface or by raypath bending that occurs at the formation interfaces subsurface. That is, the quantitative geophysical measurements tend to display the approximate topography of the irregular interface instead of the desired topography of the underlying target formation. By way of example but not by way of limitation, this disclosure will be narrated in terms of seismic exploration geophysics but the basic concepts apply equally well to other geophysical studies such as potential field or electromagnetic measurements. As is well known in seismic surveying a plurality of transducers are distributed over the surface of an area under study. A separate source, or a certain transducer selected as a source, is used for generating an acoustic wavefield while the remaining transducers serve as receivers of the acoustic waves after reflection from subsurface formations. The traveltime from the source to each of the respective receivers, corrected for non-vertical raypaths and multiplied by half the average earth velocity, is a measure of the distance between the receiver and the reflecting interface. If the reflecting interface is flat and the surface is irregular, the travel time will vary in proportion to the surface elevation, producing an incorrect image. Traditionally, to correct for surface topography, a horizontal datum is established, usually at an elevation above the tallest topographic feature. The seismic travel times are extrapolated upwards from the surface to the datum by applying a static time correction equal to the vertical distance between surface and datum divided by one-half of a preselected replacement velocity. The replacement velocity may be selected as an extension of the near-surface velocity function characteristic of the region under study. An analogous process may be implemented to correct for subsurface topography. Reduction of the seismic reflection times to a reference plane may be referred to variously as datumizing or datuming. When a large interval exists between the surface and the datum, a strictly vertical static correction path does not fit the actual ray path geometry that would have existed if the recording surface had coincided with the datum. That discrepancy is shown in Figure 1. In Figures 1 and 2 of the accompanying drawings, the ordinate is depth and the abscissa is transducer offset. Using the well-known exploding reflector concept as illustrative for a point diffractor, the raypaths such as 10 emerging at the recording surface 12 should be extrapolated upwards to the datum 13 along the dashed slant paths such as 14 of Figure 1 rather than along the dashed vertical paths such as 16 as shown in Figure 2. A pure vertical time shift as in Figure 2 simply displaces the diffraction hyperbola 18 (Figure 3B) bodily to a position deeper in time to 20 but lacking a corresponding flattening in the slopes (that is, a reduction in the normal moveout) of the limbs 22 and 24 as would be expected had the diffractor in fact existed at a greater depth. In Figures 3A and 3B, the ordinate is wavefield traveltime and the abscissa is lateral transducer location. Limb flattening is illustrated in Figure 3A which shows hyperbola 18 from Figure 3B and hyperbola 21 from a deeper bed with the expected reduction in normal moveout at limbs 23 and 25 due to the greater raypath lengths. Application of a wave-equation migration algorithm, applied from the datum, to the constant-time-displaced diffraction hyperbola 20 of Figure 3B would result in over-migration. Berryhill, in 1979, discussed the problem of wave-equation datuming in a paper entitled "Wave-equation datuming" published in Geophysics , v. 44, pp. 1329-1344 with a follow-up Note in 1984 entitled "Wave-equation datuming before stack", Geophysics , v. 49, pp.2064-2066. He provides an exact solution to the datuming problem but the main difficulty with Berryhill's method is that it is so computationally intensive as to be uneconomical. In U.S. patent 4,943,950, issued July 24, 1990, and assigned to the assignee of this invention, Beasley et al. teach a more computationally efficient method for datumizing seismic data. The '950 patent teaches a method for migrating seismic data that has been vertically static corrected. For data below a datum but above the surface elevation, a zero (or very small) migration velocity is applied. A best estimated earth velocity is used to migrate the data below the original data-recording surface. For good and sufficient reasons, use of the Beasley et al. method is confined to the use of a finite difference migration algorithm. That algorithm is a much less accurate and less efficient method than cascaded f-k migration or any algorithm that operates in the wavenumber domain such as phase-shift migration. However, algorithms that operate in the wavenumber domain are not amenable to migration from the surface using the initial zero-velocity layer of the Beasley et al. method. That is why the finite difference algorithm is required. Another such system is disclosed in Geophysics, vol. 58, No. 3, 1 March 1993, pates 393-398, LYNN W. et al, "Efficient Migration Through Complex Water-Bottom Topography", but in which before the step of time-shifting to the water bottom there is the step of time-migrating from the water surface to the water bottom, using a velocity equal to water velocity above the water bottom and equal to zero below the water bottom. After time-shifting there is a further step of time-migrating from the water bottom to the water surface using a velocity equal to zero above the water bottom and a velocity equal to the water bottom rock velocity below the water bottom. Whilst the order of the steps differs from Beasely et al, both methods use time migration (so called downward continuing). In accordance with this invention, there is provided a method for datumizing geophysical data comprising the steps of: (a) time-shifting the geophysical data from a first elevation to a second, datum, elevation; characterised by: (b) forward-modeling said geophysical data beneath said first elevation using a velocity substantially equal to zero; and (c) forward-modeling said geophysical data between said first and second elevations using a preselected replacement velocity. Thus, surface-recorded data can be projected upwards to a datum using a preselected replacement velocity that is assumed to characterize the space between the surface and the datum, it being assumed that a zero-velocity earth volume exists beneath the surface; a preselected modeling operator can then be used to extrapolate the data to the overlying datum. This datumizing method includes the steps of selecting a desired replacement velocity and then vertically time-shifting the seismic data from a surface recording elevation to a preselected datum using the selected replacement velocity. Using a desired modeling operator, the data can then be forwardly-modeled to extrapolate them to the datum using a velocity substantially equal to zero beneath the surface and using the selected replacement velocity between the recording surface and the datum. The datumized data then may be migrated using any desired migration operator. It will be seen that this can be so implemented as to give an economical and efficient datumizing method that will allow use of any desired migration algorithm for subsequent processing. In one example of this invention, the modeling operator may be a finite difference operator. For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, wherein: FIGURE 1 shows prior-art raypaths from an exploding diffractor extrapolated upwards to a datum using wave equation considerations; FIGURE 2 shows prior-art raypaths from an exploding diffractor, bent vertically upwards from a recording surface to a datum; FIGURE 3A illustrates the effect on a diffraction hyperbola of additional propagation effects caused by an incremental depth step; FIGURE 3B illustrates the effect of a constant time shift on a diffraction hyperbola; FIGURE 4 is an example of a subsurface model having a sloping recording surface; FIGURE 5 shows the seismic amplitude responses to three point diffractors at an array of receivers positioned on the sloping recording surface of FIGURE 4; FIGURE 6 displays the data of FIGURE 5 extrapolated vertically upward to a datum; FIGURE 7 is an example of f-k migration of the data of FIGURE 6 with the correct velocity yet showing overmigration; FIGURE 8 illustrates the data of FIGURE S after upward-extrapolation to the datum using forward modeling; FIGURE 9 represents the data of FIGURE 8 after application of an exemplary migration f-k operator, showing correct migration by collapsing the diffraction hyperbolas to their apices. Prior-art datumizing processes involved migration or downward continuation of data in the presence of a zero-velocity layer between the datum and the recording surface. The process of the preferred embodiment to be disclosed on the other hand involves forward-modeling or upward continuation in the presence of a zero velocity volume beneath the recording surface. Modeling is the inverse of migration. For purposes of clearly distinguishing between the two processes, three brief tutorial paragraphs follow. Modeling and its inverse, migration, both use the acoustic wave equation to describe the amplitude response P of a wavefield as a function of the recording-station offset X, the formation depth Z and the elapsed travel time T. The wavefield is assumed to be traveling in a medium (e.g. a subsurface formation) characterized by an acoustic propagation velocity V. In 2-D coordinates, the wave equation is: ∂ 2 P ∂ X 2 + ∂ 2 P ∂ Z 2 = 1 V ∂ 2 P ∂ T 2 If the wavefield characteristics are known for any two of the three coordinates X, Z or T, its attributes can be solved for the third coordinate. Given a source of energy radiating from a known point of origin at a depth Z, forward-modeling solves for the wavefield travel-time trajectories as would be seen by an array of transducers located at incremental offsets X i from the source along an arbitrary datum at Z = O. The problem may be solved using an approach, by way of example but not by way of limitation, having the form: f(P X, Z=0, T ) = ψ(Z - ΔZ), where ΔZ is a small depth step and ψ is some desired wavefield operator such as, but not limited to, a finite difference operator. Migration or downward continuation, on the other hand assumes that the travel time trajectories seen by an array of transducers located on a designated recording surface at Z = 0 are known. Migration seeks to downward extrapolate the data to evaluate Z at T = 0, using an algorithm of the form: f(P X, Z, T=O ) = (Z + ΔZ), whereis  some wavefield operator of any desired type. Refer now to Figure 4 which illustrates a subsurface model 26 containing a number of point diffractors 28, 30, 32 at depths of -250, -750 and -1250 meters respectively, referred to sea level. The diffractors lie below a recording surface 34 that slopes from +750 meters on the left to +250 meters on the right end of the section. An arbitrary datum plane 36 is established at + 1000 meters. The velocity function near the surface is 2000 meters per second (m/s). For purposes of this example, the replacement velocity between the recording surface and the datum is 2000 m/s which is a logical extrapolation of the subsurface formation velocity function. The vertical coordinate is, of course, depth while the horizontal coordinate is lateral distance, both being measured in meters. Figure 5 shows the raw diffraction hyperbolas 38, 40, 42 as recorded along the sloping recording surface 34 of Figure 4. The vertical coordinate is two-way travel time in seconds; the horizontal coordinate is lateral offset measured as a function of the separation between the respective transducers of an array. As would be expected, the left hand limbs of the hyperbolas exhibit substantially longer traveltimes by reason of the higher elevation of the recording surface. The datumizing process as shown by example here is preferably applied to stacked seismic data although it may be applied to pre-stacked data before application of DMO. It begins by applying to the travel times the usual vertical static corrections (statics) between the surface and the datum. The statics are computed by dividing the measured vertical incremental paths, such as 16 of Figure 2, by the selected replacement velocity of 2000 m/s. The hyperbolas 38, 30 and 42 of Figure 5, after vertical statics have been applied, are shown in Figure 6. The apices 38', 40' and 42' of the time-shifted hyperbolas have been pushed down by 0.5 second because of the 500-meter upward extrapolation to the datum above the apices. The right hand limbs of the hyperbolas appear to be slightly overbalanced to the right. The slopes of the limbs remain the same as before application of the statics. For that reason, the apparent moveout is too large for the traveltime window that embraces the diffractors. Therefore, any migration attempt at this point would clearly result in gross overmigration as shown by envelopes 44, 46 and 48 in Figure 7. Overmigration can be avoided by employing the following implementation of upward wavefield modeling. The data of Figure 6 are upwardly modeled using as the operator, by way of example but not limited thereto, the well-known finite-difference algorithm along with a modeling velocity V m that is characterized by where V r is the replacement velocity that is chosen as earlier explained. The subsurface earth formation is considered to be characterized by a propagation velocity approaching or substantially equal to zero and therefore the data do not undergo propagation effects during processing. Above the surface but beneath the datum, use of an exemplary modeling algorithm such as finite difference algorithm with the exemplary replacement velocity V r of 2000 m/s models the arriving wavefields to be imaged, as shown in Figure 8, by hyperbolas 50, 52 and 54. The hyperbolas appear exactly as they would had they actually been received at a recording surface 34 that in fact coincided with datum 36. Figure 9 shows the results of migrating the data-model of Figure 8 using the well-known frequency-wavenumber migration algorithm. Each of the diffraction hyperbolas 50, 52 and 54 has been properly collapsed to point sources 56, 58 and 60. The small amount of noise is due to minor limitations that are inherent in the finite-difference method that was used here for the modeling process. The exemplary processes above cited assume that the transducers are distributed over a recording surface comprising a competent rock formation. In actual fact, a highly irregular and very troublesome weathered layer exists on top of the formation. The velocity is quite low - of the order of 800-900 m/s. It is customary to arithmetically to scrape off the effects of the low-velocity layer. That is done by subtracting out the estimated travel time through the weathered layer from the recorded wavefield travel times, in a vertical sense, at each transducer. The actual or estimated travel time through the anomalous layer is based on its characteristic velocity. The wavefield travel time, thus reduced back to the top of the competent formation, is then extrapolated to the datum in the usual manner using a replacement velocity that is an extension of the formation velocity function. In marine operations, the water layer forms a low velocity layer over an irregular sea floor as earlier discussed. The effect of an irregular water thickness is removed by replacing the constant water velocity of 1500 m/s with a replacement velocity characteristic of the rocks beneath the water bottom. The preferred method in that case is to apply a downward-continuation algorithm using the constant water velocity down to the water bottom. The water bottom times are then vertically adjusted so as to place the water bottom depth at a vertical time consistent with the replacement velocity. The data are then extrapolated from the water bottom back up to the datum with a modeling operator incorporating the replacement velocity. The method disclosed herein has been explained with reference to two-dimensional (2-D) exploration techniques. Many geophysical surveys involve arrays of transducer that are spatially distributed over an area rather than along a single line. The arrays may be oriented along at least two spatial coordinates such as along northerly (in-line) and easterly (cross-line) directions or in any other desired pattern. Such three-dimensional (3-D) geophysical surveys can be processed by first forward-modeling the signals from in-line arrays followed by forward-modeling the data from cross-line arrays and combining the two sets of processed data by any desired well-known method. An efficient economical method has been disclosed for modeling geophysical data gathered from instrumentation positioned on an irregular recording surface or affected by erratic subsurface topography. The discussion is exemplary only and in no way limiting. Variations in the method will be developed by those skilled in the art but which will fall within the scope of the disclosure.

Claims: A method for datumizing geophysical data comprising the step of: (a) time-shifting the geophysical data from a first elevation to a second, datum, elevation; characterised by: (b) forward-modeling said geophysical data beneath said first elevation using a velocity substantially equal to zero; and (c) forward-modeling said geophysical data between said first and second elevations using a preselected replacement velocity. A method according to claim 1, wherein the first elevation is a surface elevation. A method according to claim 1, wherein the second elevation is higher than the first. A method according to claim 1, 2 or 3, wherein the second elevation is horizontal. A method according to any one of the preceding claims, wherein step (a) is vertical time-shifting. A method according to claim 5, wherein the step of time-shifting is accomplished by applying a time shift proportional to the vertical space existing between the first and second elevations divided by said replacement velocity. A method according to any one of the preceding claims wherein steps (a) through (c) are performed on prestack seismic data. The method according to claim 7, wherein DMO is carried out after steps (a) through (c) and stacking. A method according to any one of claims 1 to 6, wherein steps (a) through (c) are performed on stacked seismic data. A method according to claim 2, or any one of claims 3 to 9 when appended to claim 2, for datumizing seismic wavefields received by an array of transducers that are spatially distributed in an area over a recording surface providing the first elevation, the wavefields originating from within a subsurface rock formation. A method according to claim 10 when appended to claim 6, wherein the step of vertical time-shifting is accomplished by applying a time shift proportional to the vertical distance, as measured at each said transducer, between said recording surface and said datum elevation divided by said preselected replacement velocity. A method according to claim 10 or 11, wherein said preselected replacement velocity is an extrapolation of the velocity function that is characteristic of said formation. A method according to claim 10 or 11, comprising: (d) datumizing the wavefields derived from transducers aligned along a first spatial coordinate; (e) datumizing the wavefields aligned along at least a second spatial coordinate; and (f) combining the datumized wavefields from steps (d) and (e).

Inventors: MACKAY SCOTT W, MACKAY, SCOTT W.
Applicants: WESTERN ATLAS INT INC, WESTERN ATLAS INTERNATIONAL, INC.
Application year: 1994
Main classification: G01V1
IPC codes: G01V1